1. Field of the Invention
The present invention relates to a liquid crystal lens electrically driven, and more particularly, to a liquid crystal lens electrically driven having a pivot function and a stereoscopic display device using the same.
2. Discussion of the Related Art
These days, services to achieve high speed data transmission to be established based on high speed data communication networks are developed from a simply listening and speaking service, such as a current phone, toward a watching and listening service, i.e., a multi-media service through a digital terminal processing characters, voices, and images at a high speed, and is expected to be eventually developed toward a hyperspace-type real three-dimensional data communication service, i.e., a realistically and three-dimensionally watching, feeling, and enjoying service beyond time and space.
In general, a three-dimensional image is obtained by a principle of stereo vision through two eyes. Since there is parallax, i.e., a distance of about 65 cm, between two eyes, the left eye and the right eye watch slightly different images due to the positional difference between the two eyes. Such an image difference due to the positional difference between the two eyes is referred to as binocular disparity. Further, a three-dimensional stereoscopic display device enables the left eye to watch only an image corresponding to the left eye and the right eye to watch only an image corresponding to the right eye using the binocular disparity.
That is, the left/right eyes respectively watch different two-dimensional images, and when the two images are transferred to a brain through retinas, the brain correctly combines the two images and reproduces depth perception and realism of an original three-dimensional image. Such ability is usually referred to as stereography (stereoscopy), and a display device to which stereoscopy is applied is referred to as a stereoscopic display device.
Stereoscopic display devices are divided into various types according to components forming a lens implementing a three-dimensional image. For example, a type of a stereoscopic display device in which a lens is formed using a liquid crystal layer is referred to as a liquid crystal lens type electrically driven.
In general, a liquid crystal display device includes two electrodes opposite to each other and a liquid crystal layer formed between the two electrodes, and liquid crystal molecules of the liquid crystal layer are driven by an electric field generated by applying voltage to the two electrodes. The liquid crystal molecules have polarization and optical anisotropy. Here, polarization refers to change in molecular orientation according to an electric field due to concentration of electrons in liquid crystal molecules on both sides of the liquid crystal molecules when the liquid crystal molecules are placed in the electric field. Further, optical anisotropy refers to change of a path or a polarization state of emitted light according to an incidence direction or a polarization state of incident light due to a thin and long structure of the liquid crystal molecules and the above-described molecular orientation of the liquid crystal molecules.
Thereby, the liquid crystal layer represents a transmittance difference due to voltages applied to the two electrodes, and displays an image by varying the difference at respective pixels.
Recently, a liquid crystal lens electrically driven in which a liquid crystal layer serves as a lens using the above characteristics of liquid crystal molecules has been proposed.
That is, a lens controls a path of incident light at respective positions using a transmittance difference between a material forming the lens and air. If the liquid crystal layer is driven by an electric field formed in the liquid crystal layer by applying different voltages to respective positions of the electrodes, light incident upon the liquid crystal layer is changed to different phases at the respective positions, and thus the liquid crystal layer controls the path of the incident light like the actual lens.
Hereinafter, a conventional liquid crystal lens electrically driven will be described with reference to the accompanying drawings.
FIG. 1 is a longitudinal-sectional view illustrating the conventional liquid crystal lens electrically driven, and FIG. 2 is a view illustrating a potential distribution of the liquid crystal lens electrically driven of FIG. 1 after voltage is applied to the liquid crystal lens electrically driven.
As shown in FIG. 1, the conventional liquid crystal lens electrically driven includes a first substrate 10 and a second substrate 20 opposite to each other, and a liquid crystal layer 30 formed between the first substrate 10 and the second substrate 20.
Here, first electrodes 11 separated from each other by a first separation distance are formed on the first substrate 10. In the two neighboring first electrodes, a distance from the center of one first electrode 11 to the center of the other first electrode 11 is referred to as a pitch, and the first electrode 11 having the same pattern is repeated in pitch intervals.
A second electrode 21 is formed on the entire surface of the second substrate 20 opposite to the first substrate 10.
The first and second electrodes 11 and 21 are made of transparent metal. Further, the liquid crystal layer 30 is formed in a separation space between the first and second electrodes 11 and 21, and liquid crystal molecules forming the liquid crystal layer 30 have a potential surface formed in a parabolic shape due to a property reacting according to intensity and distribution of an electric field, and thus has a phase distribution similar to that of the liquid crystal lens electrically driven, as shown in FIG. 2.
Such a liquid crystal lens electrically driven is formed under the condition that high voltage is applied to the first electrodes 11 and the second electrode 21 is grounded. By the above voltage condition, the strongest vertical electric field is formed at the center of the first electrode 11, and the more distant from the first electrode 11, the weaker vertical electric field is formed. Thereby, if the liquid crystal molecules forming the liquid crystal layer 30 has positive dielectric anisotropy, the liquid crystal molecules are arranged according to the electric field such that the liquid crystal molecules are erected at the center of the first electrode 11 and are tilted nearly horizontally as the liquid crystal molecules are distant from the first electrode 11. Therefore, in terms of light transmission, as shown in FIG. 2, an optical path at the center of the first electrode 11 is short, and an optical path increases in length with increasing distance from the first electrode 11. If it is expressed on the phase surface, the liquid crystal lens electrically driven has light transmission effects similar to the lens having a parabolic surface.
Here, the second electrode 21 induces behavior of the electric field formed by the liquid crystal molecules and causes a refractive index of light to spatially form a parabolic function. The first electrode 11 forms an edge region of the lens.
A voltage value applied to the first electrode 11 is high, and thus a vertical electric field is formed between the first electrode 11 and the second electrode 21. As the distance of the vertical electric field from the first electrode 11 increases, the magnitude of the vertical electric field between the first electrode 11 and the second electrode 21 is decreased. In terms of the optical path, the liquid crystal molecules are erected at the center of the first electrode 11 and are disposed nearly horizontally as the liquid crystal molecules are distant from the first electrode 11, and thus the optical path at the center of the first electrode 11 is the shortest and the optical path increases in length with increasing distance from the first electrode 11. Therefore, the liquid crystal modules exhibit an optical path property similar to that of the lens formed in a parabolic shape at the center of the first electrode 11, thus functioning as the liquid crystal lens electrically driven.
As described above, the liquid crystal lens electrically driven is obtained by forming electrodes on both substrates disposed under the condition that a liquid crystal layer is interposed between the two substrates and then applying voltages to the electrodes without a lens having a surface formed in a parabolic shape.
However, the above-described conventional liquid crystal lens electrically driven has problems, as follows.
First, the electrode formed on the lower substrate is formed at only a part of a lens region, and a gentle electric field between a lens edge region corresponding to the electrode and a lens central region with increasing distant from the lens edge region is not formed, but a rapid lateral field is induced, thereby causing the liquid crystal lens electrically driven to have a somewhat distorted phase. Particularly, in the liquid crystal lens electrically driven, as a pitch of lens regions increases, the number of electrodes to which high voltage is applied is greatly small. Therefore, the electric field between the electrode to which the high voltage is applied in the lens region and the opposite substrate is not sufficient, and thus it is increasingly difficult to form a liquid crystal lens electrically driven formed in a gentle parabolic surface having the same effects as an actual lens.
Second, in case of a large size display device, a lens central region being increasingly distant from a lens edge region where the electrode on which high voltage is applied are located scarcely receives effects of the electric field, and thus adjustment of liquid crystal alignment at the lens central region by the electric field is difficult. In some cases, if the adjustment of liquid crystal alignment at the lens central region is impossible or difficult, the obtained liquid crystal lens electrically driven has a discontinuous lens profile, thus being incapable of being used as a lens.
Third, the electrodes of the liquid crystal lens electrically driven are formed in one direction, and thus the lens functions only in one direction. Therefore, if a display panel is rotated by an angle of 90°, the electrodes of the liquid crystal lens electrically driven cannot display a three-dimensional image. Accordingly, an effort to make a liquid crystal lens electrically driven which, even if the display panel is rotated, can function in a rotated direction, and a stereoscopic display device having the above function has been proposed.